The present invention pertains to fluid delivery devices, and, in particular, to a sensing system for determining the position of a plunger within a fluid container.
A variety of known types of devices used to deliver fluid medication to a patient utilize medication-filled containers having movable plungers, such containers including cartridges and syringes. These types of devices include, but are not limited to, infusion pumps such as insulin pumps and medication injectors such as injection pens. A medication cartridge includes a movable plunger that seals medication within a barrel of the cartridge forward of the plunger. The cartridge plunger, when advanced by a drive mechanism of the delivery device in which the cartridge may be installed, forces medication from the cartridge through an outlet of that cartridge for delivery to a user.
Knowing the axial position of a plunger within a container allows for various types of information to potentially be available to a user. For instance, the absolute position of the plunger may allow a determination of the quantity of medication remaining in the container. In addition, changes in axial position of the plunger may allow a determination of a dose dispensed from the container.
A variety of systems previously have been developed to determine in some fashion the position of a plunger within a container. One such type of system uses optic properties. U.S. Pat. No. 6,113,578 discloses a number of designs that permit optical dose measurements in syringes. While potentially useful, these designs are not without their shortcomings. For instance, these designs are relatively complex in that they require a significant number of operative components. This complexity can result in greater costs associated with manufacture. Still further, this complexity may increase the number of potential failure modes, as well as require more space than is desired when incorporated in devices that are desired to be kept small, compact and portable.
Thus, it would be desirable to provide a plunger sensing system, or a device that employs such a sensing system, that can overcome one or more of these and other shortcomings of the prior art.